1. Field of the Invention
The present invention relates to a method for a selective isolation of m-dichlorobenzene from an isomeric mixture thereof. More particularly, the present invention relates to a method for selectively isolating m-dichlorobenzene, which is useful as an intermediate of pesticides (agricultural chemicals), medicines and dyestuffs, from an isomeric mixture thereof by using an absorbing material.
2. Description of the Related Arts
It is known that m-dichlorobenzene can be prepared by the chlorination of benzene or chlorobenzene or by the isomerization of o- and/or p-dichlorobenzene. The resultant product is in the form of an isomeric mixture of m-dichlorobenzene and o- and/or p-dichlorobenzene.
It is also known that o-dichlorobenzene (o-DCB) has a boiling point of 180.4.degree. C., m-dichlorobenzene (m-DCB) has a boiling point of 173.0.degree. C., and p-dichlorobenzene (p-DCB) has a boiling point of 174.1.degree. C.; namely, these materials have boiling points which are close to each other.
In the dichlorobenzene isomers, the o-isomer exhibits a higher melting point than those of m- and p-isomers, and thus can be isolated and collected by a rectification column having a high stage number. However, it is very difficult to completely isolate the o-isomer from the m- and p-isomers by a high stage rectification.
Japanese Unexamined Patent Publication No. 50-19722 discloses a method for separating m- and p-DCB isomers. In this method, the m-isomer is isolated by an extract-distillation in the presence of a natural polar solvent, for example, hexamethylphosphotriamide or dimethylsulfoxide, and then the p-isomer extracted in the natural polar solvent is isolated from the natural polar solvent solution by distillation thereof.
However, the extract distillation method is disadvantageous in that the yield and purity of the isolated m-isomer are unsatisfactory.
In another separating method, the isomeric mixture is subjected to a sulfonation procedure and then m-dichlorobenzene-4-sulfonic acid is isolated from the sulfonation product by crystallization deposition.
This method is disadvantageous in that the purity of the resultant isolated m-isomer is not satisfactorily high and the isolation procedures are costly.
Still other separating methods, in which various faujasite types (X or Y type) of zeolites are used as an absorbing material to isolate the m-DCB isomer as a raffinate or extract fraction from a DCB isomeric mixture, are disclosed in Japanese Examined Patent Publication No. 37-5155 and Japanese Unexamined Patent Publication Nos. 52-62229, 53-105434, 58-131,924, 58-150,524, and 61-268,636.
However, the conventional faujasite type zeolite does not effect a satisfactory isolation of the m-isomer and, therefore, it is practically impossible to obtain a high purity m-isomer. Furthermore, the conventional isolating method in which the faujasite type zeolite is used as an absorbing material is disadvantageous in that the isolation of the m-isomer must be carried out in the presence of an eluent, and the m-isomer must be separated from the eluent by distillation of the eluent m-isomer solution.
Under the above-mentioned circumstances, there is a need for a new m-DCB isomer-isolating method which can be effected at a high efficiency by a simple and easy operation.